1. Field of the Invention
This invention relates to the field of high speed electronic circuits, such as shift registers, and more particularly to a MESFET signal switch and input MESFET logic element circuit.
2. Description of the Prior Art
Metal-semiconductor field-effect transistors (MESFETs) have been known at least since the mid-1960's. Gallium arsenide (GaAs) metal-semiconductor field-effect transistors (MESFETs) are also known and are described in various publications, such as S. Y. Liao, Microwave Devices and Circuits, Prentice-Hall, Inc., pp. 288-300.
Shift registers of different types are generally known and described in the literature. Dynamic shift registers frequently consist of a master-slave circuit where both master and slave circuits are identical and consist of a switch and an inverting amplifier, and where the output of the master circuit is the input to the slave circuit. Dynamic shift registers using metal-oxide semiconductor field-effect transistors (MOSFETs) have been demonstrated as shown in R. H. Crawford, MOSFET in Circuit Design, McGraw-Hill, Inc., pp. 107-112.
MOSFET shift registers have produced relatively fast shift registers, but it is desirable to constantly search for still faster shift rates. Shift registers using MOSFETs are slowed in two ways. First, the circuits used to shift information involve several gate delays per shift operation. Second, MOSFETs are, of themselves, slower operating than other electronic switches, such as MESFETs. This is a result of a charge storage phenomenon associated with the gate electrode and SiO.sub.2 gate oxide of MOSFET devices. This phenomenon is not encountered with MESFET devices due to the use of a Schottky barrier gate electrode.
Therefore, since MESFETs, MESFET shift registers, and other logic circuits implemented with MESFETs are inherently faster than their MOSFET counterparts, attempts have been made to construct high speed MESFET digital circuits without the use of any MOSFET devices. Certain of these circuits require the use of a transmission or input signal switching gate. Such a gate is typically used to selectively enable the transmission of a signal serially through the gate to a subsequent portion of the circuit. However, standard depletion mode MESFET devices, when used as a transmission gate, normally cannot be maintained in a low impedance state for both "one" and "zero" logic levels without forward biasing the gate of the MESFET. Thus, these devices have not been suitable as input switching devices for MESFET circuits since, under such forward biased conditions, this will inevitably introduce undesirable errors into the intended operation of the circuitry. Consequently, prior art MESFET circuits normally requiring the use of an input switching or transmission gate will use an input MOSFET switching device or the like to alleviate this problem. At the same time, however, the inherent speed limitation of MOSFET devices is imposed on the entire remaining MESFET circuitry with a corresponding loss of the speed advantage of using MESFETs.